Transimpedance amplifiers are employed in optical receivers in order to convert very small currents indicative of optical signals applied to photodiode detectors. These small currents are then converted to signal voltages of greater amplitude. In an optical fiber communications network whereby a plurality of geographically distributed users each write onto a common optical fiber, incoming optical signals from a nearby transmitter may be detected at a high signal level, whereas incoming optical signals received from a distant transmitter may be detected at very low signal levels. Thus, to be effective, a transimpedance front end of the optical receiver must be sufficiently sensitive effectively to receive the weakest optical signals and must also be controllable to receive the strongest optical signals without reaching saturation and resultant distortion in the detected signal voltages.
High sensitivity and high saturation level are contradictory requirements for a transimpedance front end of an optical receiver. A simplified representation of a typical transimpedance front end of an optical receiver is illustrated in FIG. 1. With reference to the FIG. 1 configuration, if the amplification factor -A is sufficiently large, the transimpedance is equivalent to the feedback resistance R.sub.L. On one hand, for high sensitivity the R.sub.L value should be large because the noise current introduced by the feedback resistor is inversely proportional to the resistor value. On the other hand, to realize a high saturation level, the value of R.sub.L should be small in order to limit signal excursion.
Techniques reported in the prior art for increasing dynamic range improvement typically involve the use of active devices at sensitive nodes of the transimpedance amplifier. One example is given in FIG. 2. In FIG. 2, an FET device is provided at the input of the amplifier to shunt away photo current at high signal levels to prevent saturation of the amplifier. In FIG. 3, an FET device is shunted across a portion R.sub.L1 of the feedback resistor R.sub.L in order to lower the value thereof at high signal levels. The FET devices employed in these prior art examples are placed at sensitive nodes, i.e. the input of the amplifier in FIG. 2 and across the feedback resistor in FIG. 3. The FET devices add parasitic capacitances into the amplifier circuit, and these parasitics have a significant effect upon the performance of the overall amplifier circuit. In order to minimize the effect of the parasitics, the characteristics of the FET devices must be chosen carefully and tightly controlled within a narrow tolerance. Usually, the FET devices have been incorporated into an integrated circuit amplifier wherein the design of the active device may be customized for a particular application.
Neither of the techniques illustrated in FIGS. 2 or 3 for extending the dynamic range of an optical receiver is convenient if the designer is limited to standard "off the shelf" discrete circuit components and elements.